Helen Mirren stars in Eye in the Sky, which features a version of this classic philosophical question.
Photograph: Courtesy of Entertainment One UK

In the 2015 British thriller Eye in the Sky, a military team locates a terrorist cell preparing an attack expected to kill hundreds. They command a drone that can drop a bomb on the terrorists, preventing their attack. As the team readies the bomb, their cameras spy a little girl selling bread within the blast radius. Should they go through with their mission – killing the girl in order to prevent the deaths of many others?

This modern-day moral dilemma has its roots in a classic philosophical thought experiment known as the trolley problem. Introduced in 1967 by Philippa Foot, the trolley problem illuminates the landscape of moral intuitions – the peculiar and sometimes surprising patterns of how we divide right from wrong.

Try it at home

Consider one version of the trolley problem:

A runaway trolley is heading down the tracks toward five workers who will all be killed if the trolley proceeds on its present course. Adam is standing next to a large switch that can divert the trolley onto a different track. The only way to save the lives of the five workers is to divert the trolley onto another track that only has one worker on it. If Adam diverts the trolley onto the other track, this one worker will die, but the other five workers will be saved.

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Should Adam flip the switch, killing the one worker but saving the other five? Write down your answer.

Now consider a slightly different version:

A runaway trolley is heading down the tracks toward five workers who will all be killed if the trolley proceeds on its present course. Adam is on a footbridge over the tracks, in between the approaching trolley and the five workers. Next to him on this footbridge is a stranger who happens to be very large. The only way to save the lives of the five workers is to push this stranger off the footbridge and onto the tracks below where his large body will stop the trolley. The stranger will die if Adam does this, but the five workers will be saved.

Should Adam push the stranger off the footbridge, killing him but saving the five workers?

Did you give the same answer to the first and second versions – or different ones?

What’s going on?

The trolley problem highlights a fundamental tension between two schools of moral thought. The utilitarian perspective dictates that most appropriate action is the one that achieves the greatest good for the greatest number. Meanwhile, the deontological perspective asserts that certain actions – like killing an innocent person – are just wrong, even if they have good consequences. In both versions of the trolley problem above, utilitarians say you should sacrifice one to save five, while deontologists say you should not.

Psychological research shows that in the first version of the problem, most people agree with utilitarians, deeming it morally acceptable to flip the switch, killing one to save five. But in the second version of the problem, people lean deontological and believe it’s not acceptable to push a stranger to his death – again killing one to save five. What can explain this discrepancy?

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Scientists think that our moral intuitions evolved to make us good social partners. Because we learn from a very young age that violence towards others is typically punished, our moral intuitions tell us it’s wrong to take actions that physically harm others. So in versions of the trolley problem that involve physical contact, like the footbridge case above, harming one to save many is generally less acceptable than in versions that do not involve such contact, like the switch case.

Another crucial difference between the switch case and the footbridge case is that the latter involves using a person as a means to an end. Treating others as individuals with their own rights, wishes and needs, rather than simply objects to be used at will, is a key aspect of being a good social partner. And there is evidence that people strongly distrust those who use others as a means to an end. Our moral intuitions seem to accord with this principle.

Critics of the trolley problem say it is too unrealistic to reveal anything important about real-life morality. But the rise of drones and self-driving cars makes the dilemma perhaps more relevant than ever before. For example, should a self-driving car protect the life of its passengers, even at the expense of a greater number of pedestrians? Here too, our intuitions are inconsistent: we want other people’s cars to maximize the number of lives saved – but think our own car should protect us at all costs. As our technologies become increasingly capable of making moral decisions, understanding our own moral intuitions becomes all the more crucial.